The present invention relates in general to data communication systems and more particularly to a precursor decision feedback equalizer (DFE) capable of canceling precursor Inter-Symbol Interference (ISI) even when future data is not available.
In data transmission along Unshielded Twisted Pair (UTP), the transmission channel is never perfect. Rather, the channel pulse response is characterized by a rising edge and a long tail. As a result, samples received over the UTP transmission channel are known to contain interference from all of the data transmitted. Such interference is called Inter-Symbol Interference (ISI). Decision Feedback Equalizers (DFEs) are known in the art for combating ISI. These DFEs are incorporated into prior art digital data receivers.
The purpose of a DFE is firstly to estimate the contributions of non-current data relative to the current received data, and then to subtract these contributions to cancel the ISI. After the ISI has been canceled, the current data can be detected with a simple threshold detector. A most commonly used algorithm for the DFE is the (Least Mean Square) LMS adaptive algorithm, such as described in xe2x80x9cC261(UNIC) DSP Re-engineering and Performance Reportxe2x80x9d Mitel Semiconductor, Document No. C261AP13, Oct. 21, 1996). During a so-called training period of the DFE, all of the data is known, and the contributions to the ISI can be estimated with an adaptive algorithm. Then, during normal transmission, post-estimated data can be used in the adaptive DFE to cancel the postcursor ISI (i.e. ISI caused by data already received). However, since xe2x80x9cfuturexe2x80x9d data is not available, it is difficult to eliminate precursor ISI (i.e. ISI caused by future bauds in the channel which have not yet been received by the receiver). As a result, the data detection performance is deteriorated in prior art DFEs due to the existence of the precursor ISI.
According to the present invention, a new DFE structure is provided which cancels the first precursor ISI. In almost all applications, the primary contribution to precursor ISI comes from the next nearest baud. In other words, only one precursor symbol is significant. Therefore, data detection performance can be dramatically improved by the elimination of only the first precursor ISI.
In prior art DFE algorithms, the sampling location is established such that the first precursor is small enough to be ignored. As a result, the main tap sampling location is normally located at the rising edge of the channel, which may not be at the peak location of the channel, and therefore the received SNR (i.e. the signal-to-noise ratio between the main tap weight and the noise level) suffers a loss. By allowing for the existence of the first precursor and ISI cancellation based thereon, the main tap weight for the current data can be moved further toward the peak location of the channel response in the DFE implementation of the present invention, such that the SNR is maximized. Consequently, the maximum loop length can be reached for transmission over unshielded twisted pair cable. This is useful for Basic Rate ISDN (2B1Q) transceivers as well as higher speed transceivers such as HDSL.